[Local] Add dbg location on unreachable inst in changeToUnreachable

As show in https://bugs.llvm.org/show_bug.cgi?id=37960
it would be desirable to have debug location in the unreachable
instruction.

Also adds a unti test for this function.

Differential Revision: https://reviews.llvm.org/D50340

llvm-svn: 339173